(a) Field of the Invention
The present invention relates generally to a liquid crystal display and a driving method thereof, and more particularly to a liquid crystal display for wide viewing angle for suppressing occurrence of lower gray level inversion and a driving method thereof.
(b) Description of the Related Art
In general, the reason that a lower gray level inversion occurs in TN (twisted nematic) type LCD (liquid crystal display) is as follows. For the convenience of description, an ECB (electrical controlled birefringence) mode will be given. For an LCD of ECB mode, rubbing directions of lower and upper alignment films are equal or opposite each other, a twist angle is 0°, transmission axes of a polarization plate and a light-detection plate are perpendicular to each other, and transmission axis of the rubbing direction has an inclination of 45° with respect to the rubbing direction.
When each of three voltages, V1, V2, and V3 (V1<V2<V3) is applied to liquid crystal cells, liquid crystal directors are arranged as shown in FIG. 1.
FIG. 1 is a view of liquid crystal directors dependent on voltages applied to the liquid crystal cells.
As shown in FIG. 1, since a phase retardation by the liquid crystal is decreased with increase of an application voltage when light is perpendicular to a plane of the liquid crystal cell array, light can not pass through the liquid crystal cell if polarization plates are placed perpendicular to each other at lower and upper portions of the liquid crystal cells. In other words, the higher the voltage is, the lower the transmission rate is.
However, when light is incident a certain inclination angle with respect to the plane of the liquid crystal cell array, the transmission rate is decreased with gradual decrease of the phase retardation when the application voltage rises from V1 to V2 but is increased with gradual increase of the phase retardation when the application voltage rises from V2 to V3.
In other words, the transmission rate is high at a higher application voltage rather than a lower application voltage above a certain angle. This is referred to as “gray level inversion”, which will be explained with reference to FIG. 2.
FIG. 2 illustrates a gray level indication according to a prior viewing angle.
Referring to FIG. 2, a normal gray level can be identified in the front sight of the liquid crystal panel, but an abnormal gray level may be identified in the sight from a position lower than the front. In other words, when the panel is observed above a certain angle in the sight from a position lower than the front, there is a problem of a lower gray level inversion that it is perceived that white gray level is inverted to black gray level and conversely.
Such a lower gray level inversion causes a problem of narrow viewing angle that viewing angle of the liquid crystal display becomes narrow.
One approach for solving the narrow viewing angle problem is to use a compensation film. However, this approach is excellent in improvement effect of CR (contrast ratio) but has a problem that gray level property is little improved.
In addition, another approach for solving the narrow viewing angle problem is to use an IPS (in plane switching) mode or a VA (vertical alignment) mode. However, this approach requires a complex process and has a problem of poor yield.
In the other hand, a flicker occurs in the liquid crystal display due to a swing of a common electrode voltage or a difference of response time of the liquid crystal. These reasons of occurrence of the flicker will be described with reference to FIGS. 3a, 3b, and 4 of the accompanying drawings.
Firstly, FIGS. 3a and 3b illustrate a flicker caused by a swing of common electrode voltage generated in a prior liquid crystal display. With reference to these figures, the liquid crystal display with a normal white mode, which has white gray level in case of no application of voltage to the pixel and black gray level in case of application of voltage to the pixel, will be described as an example.
More particularly, FIG. 3a shows pixel voltages applied to first to fourth pixels for each frame.
Referring to FIG. 3a, although a pixel application voltage should be applied around an ideal common electrode voltage (Ideal Vcom), since a common electrode voltage (Actual Vcom) is shifted by a certain level at the time of actual driving, the magnitude of the pixel voltage applied to the first frame becomes different from that of the pixel voltage applied to the second frame to thereby generate the flicker.
FIG. 3b shows pixel voltages actually felt by pixels, which are applied to the first to fourth pixels placed spatially in FIG. 3a for each frame.
Referring to FIG. 3b, as the second and third frames have brightness of (L−) and (H′+) in the entire screen and the first and fourth frames have brightness of (H−) and (L−), a brightness difference between the two brightness produces a flicker of 15 Hz component.
FIG. 4 is a diagram of a flicker caused by a difference of response time of the liquid crystal generated in a prior liquid crystal display, particularly (a) is for illustrating a voltage applied to a certain pixel for each frame (7 frames shown) and a brightness level responding to the voltage and (b) is for illustrating a voltage applied to a pixel adjacent to the certain pixel for each frame and a brightness level responding to the voltage.
Referring to FIG. 4, due to a difference between response time of when a low voltage is changed to a high voltage and that of when a high voltage is changed to a low voltage, a flicker occurs in a portion indicated by a circle in the entire screen when the pixels having two waveforms in the right and left are driven on their average.